The present invention relates generally to power system communications, and more particularly to apparatus capable of simultaneously transmitting and receiving digital data signals both at high rates and over long distances through power lines and power line transformers, including AC, DC, coaxial cables, and twisted pair lines.
xe2x80x9cPower-line Carriersxe2x80x9d are well known in the field-of power system communications. The principal elements of such power-line carriers are transmitting and receiving terminals, which include one or more line traps, one or more coupling capacitors, and tuning and coupling equipment. Detailed information regarding the description and typical composition of conventional power-line carriers may be found in Fundamentals Handbook of Electrical and Computer Engineering Volume II: Communication Control Devices and Systems, John Wiley and Sons, 1983, pp 617-627, the contents of which are incorporated herein by reference. A significant problem associated with prior art power-line carriers is their requirement for one or more line traps, one or more capacitors, one or more coupling transformers or carrier frequency hybrid circuits and frequency connection cables.
All traditional couplers incorporate a ferrite or iron core transformer which causes signal distortion due to the non-linear phase characteristic of the transfer function between the transmit coupler and the receive coupler. The distortion is created by the presence of magnetic core material which exhibits hysteresis. For distribution power-line carriers, the distortion -is particularly severe because the signal must propagate through at least three such non-linear devices, the distribution transformer and two power-line couplers, that use ferrite core transformers. The distortion caused by these non-linear devices leads to envelope delay distortion, which limits communication speeds.
A line with a characteristic impedance Zo is ideally matched by terminations equal to Zo at both ends. Since Zo is primarily resistive at the frequencies of interest, the input impedance of the couplers should also be primarily resistive and equal to Zo at the carrier frequencies. A general configuration to achieve this is shown in FIG. 4. FIG. 4 is a schematic diagram of a phase shift linear coupler of the present invention. The coupler uses a serially connected equivalent capacitor, Ceq, on the primary of a transformer. The design is based on two principles. First, the resonance between the coupling capacitor, Ceq, and the primary winding inductance, L1, provides a low resistive impedance at the desired transmit carrier frequency. Second, Ceq has a large enough impedance at 60 Hz to block the line frequency. Although this basic approach is not new, previous efforts at achieving satisfactory impedance matching encountered problems, as discussed below.
The major shortcoming of previous designs resulted from the use of ferrite or-iron core transformers in the signal couplers. The inductance, L1, is altered to some unknown value due to the non-linearity of the core. This results in a mistuning of the desired carrier frequency. Also, the impedance of the primary winding at the desired carrier frequency is no longer purely resistive. This may lead to a mismatch with respect to the line characteristic impedance. In recognition of this fact, other designs (FIGS. 1, 2) attempt to merely couple a signal onto a power line with a low transceiver input impedance by using a large coupling capacitor (approx. 0.5 uF). This results in a significant coupling loss of up to 20 dB at the carrier frequency. FIG. 3 is a graphical illustration of the frequency response characteristics of a traditional coupler which uses a magnetic-core transformer.
In view of the above, it is an object of the present invention to provide a power line communications apparatus which utilizes a novel phase shift linear power, phone, twisted pair, and coaxial line coupler for both transmission and reception. It is a further object of the present invention to provide power-line communication apparatus utilizing novel air-core transformers which can be used for phone line, coaxial, LAN and power line communication through power line transformers. It is an additional object of the present invention to provide a power line communication apparatus in which the primary coil of the transformer resonates with an associated coupling capacitor network in order to achieve resistive matching to approximately the lowest known value of the line characteristic impedance and to maximize stable signal transmission onto the line. This resonance effectively creates a band pass filter at carrier frequency.
Briefly stated, in a first embodiment, the present invention is a communications apparatus for communicating electrical signals through an electrical line having a characteristic impedance. The communications apparatus comprises:
modulator means for modulating the electrical signals to produce a modulated carrier signal having a first preselected frequency;
transmitter means having an output impedance, electrically connected to said modulator means for transmitting the modulated carrier signal; and
first coupler means connected between the electrical line and the transmitter means for matching the output impedance of said transmitter means to the characteristic impedance of the electrical line, wherein the first preselected frequency is less than 90 Khz and has a linear phase bandwidth of approximately 7 Khz.
In a second embodiment, the present invention is a communications apparatus for communicating electrical signals through an electrical line having a characteristic impedance. The apparatus comprises:
modulator means for modulating the electrical signals to produce a modulated carrier signal having a first preselected frequency;
transmitter means having an output impedance, electrically connected to said modulator means for transmitting the modulated carrier signal;
first coupler means connected between the electrical line and the transmitter means for matching the output impedance of said transmitter means to the characteristic impedance of the electrical line, said first coupler means comprising linear phase means for communicating the modulated carrier signal to the electrical line without significant phase distortion and capacitor means for resonating with the linear phase means at the first preselected frequency;
receiver means having an input impedance for receiving the modulated carrier signal;
demodulator means electrically connected to said receiver means for demodulating said modulated carrier signal to produce a demodulated carrier signal having-a second preselected frequency; and
second coupler means connected between the electrical line and the receiver means for matching the input impedance-of said receiver means to the characteristic impedance of the electrical line, said second coupler means comprising linear phase means for communicating the modulated carrier signal to the receiver means without significant phase distortion and capacitor means for resonating with the linear phase means at the second preselected frequency.
In a third embodiment, the present invention is a communication apparatus for communicating electrical signals through a pair of electrical lines having a characteristic impedance. The apparatus comprises:
first modem means for producing a modulated carrier signal having a first preselected frequency and demodulating a modulated carrier signal having a second preselected frequency;
first transmitter means having an output impedance, connected to the first modem means for transmitting the modulated carrier signal at the first preselected frequency;
first receiver means having an input impedance, connected to the first modem means for receiving the modulated carrier signal having the second preselected frequency;
first coupler means connected between the pair of electrical lines and said first transmitter and receiver means for matching the impedance of said means to the characteristic impedance of the pair of electrical lines and for communicating the carrier signals without substantial phase distortion;
second modem means for producing a modulated carrier signal having the second preselected frequency and demodulating a modulated carrier signal having the first preselected frequency;
second transmitter means having an output impedance, connected to the second modem means, for transmitting the modulated carrier signal at the second preselected frequency;
second receiver means having an input impedance connected to the second modem means, for receiving the modulated carrier signal having the first preselected frequency; and
second coupler means connected between the electrical lines and said second transmitter and receiver means for matching the impedance of said devices to the characteristic impedance of the electrical lines and for communicating the carrier signals without substantial phase distortion.
In a fourth embodiment, the present invention is a communication apparatus for communicating electrical signals through a pair of electrical lines having a characteristic impedance. The apparatus comprises:
first modem means for producing a modulated carrier signal having a first preselected frequency and demodulating a modulated carrier signal having a second preselected frequency;
first transmitter means having an output impedance, connected to the first modem means, for transmitting the modulated carrier signal at the first preselected frequency;
first receiver means having an input impedance, connected to the first modem means, for receiving the modulated carrier signal having the second preselected frequency;
first coupler means connected between the pair of electrical lines and said first transmitter and receiver means for matching the impedance of said first transmitter and receiver means to the characteristic impedance of the pair of electrical lines, said first coupler means comprising two LCR circuits, each of the LCR circuits comprising at least one capacitor and at least one resistor connected in parallel to each other and in series to the pair of electrical lines and a linear phase means for communicating the carrier signals without significant phase distortion;
second modem means for producing a modulated carrier signal having the second preselected frequency and demodulating a modulated carrier signal having the first preselected frequency;
second transmitter means having an output impedance, connected to the second modem means, for transmitting the modulated carrier signal at the second preselected frequency;
second receiver means having an input impedance, connected to the second modem means, for receiving the modulated carrier signal having the first preselected frequency;
second coupler means connected between the pair of electrical lines and said second transmitter and receiver means for matching the impedance of said second transmitter and receiver means to the characteristic impedance of the pair of electrical lines, said second coupler means comprising two LCR circuits, each of the LCR circuits comprising at least one capacitor and at least one resistor connected in parallel to each other and in series to the pair of electrical lines and a linear phase means for communicating the carrier signals without significant phase distortion.
In yet a fifth embodiment, the present invention is a communications apparatus for communicating electrical signals through an electrical line having a characteristic impedance comprising:
modulator means for modulating the electrical signals to produce a modulated carrier signal having a preselected frequency;
transmitter means having an output impedance, connected to the modulator means for transmitting the modulated carrier signal;
first coupler means connected between the electrical line and the transmitter means for matching the output impedance of the transmitter means to the characteristic impedance of the electrical line, the first coupler means comprising linear phase means for communicating the modulated carrier signal to the electrical line without substantial phase distortion and capacitor means for resonating with the linear phase means at the preselected frequency;
demodulator means for demodulating the carrier signal on the electrical lines to produce a demodulated carrier signal having the preselected frequency;
receiver means having an input impedance connected to the demodulator means for receiving the modulated carrier signal;
second coupler means connected between the electrical line and the receiver means for matching the input impedance of the receiver means to the characteristic impedance of the electrical line, the second coupler means comprising linear phase means for communicating the modulated carrier signal to the receiver means without substantial phase distortion and capacitor means for resonating with the linear phase means at the preselected frequency, wherein the linear phase means for each of the first and second coupler means comprises air-core transformer means comprising a primary coil having a first diameter, a secondary coil having a second smaller diameter, the secondary coil extending coaxially within the primary coil such that an air-gap is created between the primary and the secondary coils, and capacitor means connected between the primary coil and the electrical line wherein the primary coil and the capacitor means are matched to the characteristic impedance of the electrical line at a preselected bandwidth, wherein the primary coil includes a resistive component, and wherein for half duplex communications, the resistive component of the primary coil connected to the transmitter means is around 1 ohm and the resistive component of the primary coil connected to the receiver means is around 1 ohm.
In a sixth embodiment, the present invention is a communication apparatus for communicating electrical signals through an electrical line having a characteristic impedance, the apparatus comprising:
modulator means for modulating the electrical signals to produce a modulated carrier signal having a preselected frequency;
transmitter means having an output impedance, connected to the modulator means for transmitting the modulated carrier signal; and
first coupler means connected between the electrical line and the transmitter means for matching the output impedance of the transmitter means to the characteristic impedance of the electrical line, the first coupler means comprising linear phase means for communicating the modulated carrier signal to the electrical line without substantial phase distortion and capacitor means for resonating with the linear phase means at the preselected frequency.